How do I control bagworms on my spruce tree?


How do I control bagworms on my spruce tree?


Bagworms are caterpillars that live inside spindle-shaped bags.  (The bags somewhat resemble small Christmas tree ornaments hanging from the tree.)  Bagworms feed on the foliage of a wide variety of trees and shrubs.  However, they are most commonly found on juniper, arborvitae, spruce, and other evergreens.  The spindle-shaped bags are made of silk and bits of foliage (needle) fragments.  The bags protect the caterpillars from their natural enemies.  Bagworms are most often found in the southern half of Iowa. 

Bagworms over-winter in the egg stage inside female bags attached to plants.  In Iowa, the eggs typically hatch in early to mid-June.  Immediately after hatching, the small caterpillars begin to feed and construct the bags in which they live.  The caterpillars are initially about 1/4 inch long, but they grow rapidly as they feed on plant foliage.  As the caterpillars grow, they continuously enlarge their bags by adding fragments of foliage or needles to the structures.  Feeding continues until mid to late August.  At this time, the bags are approximately 2 inches long and the caterpillars 1 inch in length.  The mature caterpillars pupate inside the bags.  The adult moths emerge in fall.  Female moths are wingless, have no functional legs, and never leave their bags.  Males are small, black, winged moths.  The males fly to the females and mate through the bag entrances.  The fertilized females lay their eggs inside the bags.  The eggs remain in these bags until they hatch the following year. 

Light infestations of bagworms can be controlled by handpicking the bags from infested plants and destroying them.  The bags should be removed before the eggs hatch in June.  Insecticides can also be used to control bagworms.  Insecticides should be applied within a few days of egg hatch.  Effective insecticides include Bacillus thuringiensis (Dipel, Thuricide, etc.), Sevin, and permethrin (Eight).  Chemical control becomes less effective as the season progresses because of the increased size of the larva and its bag.  Spray treatments made after mid-July are usually ineffective. 

Last updated on
March 3, 2022